Backup / Restore w/o XATTRs

Building on the new XATTRS fields added with MB-21941 and their inclusion into cbbackupmgr MB-23861 It would be useful to have a method to backup a bucket and exclude the XATTRS and restore a bucket from a backup that has XATTRS and exclude them on restore.

There's a number of situations where this would be helpful,

1)  If for whatever reason the data stored in the bucket was no longer needed for Mobile or whatever the XATTRS were added for, there's a clean method to strip them from all the documents.

2)  Customers may want to create a smaller backup size to deploy onto dev/test environments where the XATTRS are not needed.

3) There may arise a situation where the XATTRS are corrupt and a customer needs a method to recover the Key/Value data alone to recover the situation.